Monotube strut with rebound cut-off feature

ABSTRACT

A suspension damper including a tube and a damping piston assembly disposed within the tube and slidably mounted therein for reciprocal movement in the tube. The suspension damper also includes a piston rod extending through the tube and connected to the damping piston assembly, a rod guide assembly closing a bottom end of the tube, and a rebound cut-off disk and spring suspended in the tube between the rod guide assembly and the damping piston assembly and cooperating with the damping piston assembly to provide a rebound cut-off effect between the rebound cut-off disk and the damping piston assembly. The rebound cut-off disk and spring have specific gravities that are greater than the specific gravity of the fluid in the tube, such that the disk and the spring sink in the fluid.

TECHNICAL FIELD

The present invention relates to a suspension damper with reboundcut-off for use in a vehicle suspension system and, more particularly,to a suspension damper with a hydraulic rebound cut-off feature thatprovides a hydraulically cushioned stop at an end of rebound travel inthe damper.

BACKGROUND OF THE INVENTION

A damper operates in vehicle suspensions as a damping device controllingthe sprung (body) and unsprung (wheels) masses of a vehicle by reducingloads or vertical accelerations normally transmitted from the wheels tothe body. Damping is accomplished by converting kinetic energy intothermal energy and dissipating the heat. Conventionally, hydraulicdampers include a piston with a connected piston rod slidably carried ina fluid-filled tube and separating the tube into extension andcompression chambers. A rod guide at the top end of the tube closes theextension chamber and slidably engages the piston rod. As the parts ofthe vehicle suspension to which the cylinder tube and piston rod areattached move relative to one another, the damping piston assembly ismoved in compression and rebound strokes along the axis of the damper.The damping piston assembly includes passages, and in some dampers alsospecial valve arrangements associated with the passages, that allowfluid in the working chamber to flow through the damping piston assemblyat a controlled rate to provide damping of the relative motion betweenthe parts of the vehicle suspension to which the damper is attached.

In many applications, the suspension damper is called upon to limit thefull extension travel of the suspension system. In some prior dampers,mechanical rebound stops that are fixed to the piston rod and engageablewith the rod guide are known to provide a means of limiting the maximumextension travel of the piston rod from the damper. A typical mechanicalrebound stop is generally equipped with a resilient bumper made ofmaterial such as rubber or urethane. The bumper is designed to cushionthe engagement of the damping piston with the rod guide at the end ofdamper travel in the extension direction. This type of a mechanical stoptends to result in somewhat of an abrupt means of limiting travel duringrebound. It has also been found that, in severe applications, aresilient bumper material may undesirably experience heat degradationwhen the bumper absorbs the entire rebound stop load.

It has also been the practice in some prior hydraulic dampers to provideelements attached to the damping piston assembly and the cylinder tubethat provide additional hydraulic damping force acting against thepiston during a portion of the rebound stroke, for slowing the dampingpiston assembly as it approaches the end of the rebound stroke. Thisfunction of providing additional damping at the end of the reboundstroke, for slowing the rate of rebound, is also known as hydraulic“rebound cut-off.” Examples of this approach are disclosed in U.S. Pat.No. 6,209,691 to Fehring et al. and U.S. Pat. No. 5,706,920 to Pees etal., and in British Patent No. 691,477 to Stephens.

In recent years, hydraulic dampers using a special type of fluid, knownas Magneto-Rheological (MR) fluid, have been utilized as part of vehicletraction and stability enhancement control systems for activelycontrolling the amount of damping provided under varying road andoperating conditions to provide improved performance and safe operationof vehicles. An MR fluid is generally significantly more viscous and hasa higher specific gravity than the hydraulic fluids used in priorvehicle dampers. As a result, elements for providing a hydraulic reboundcut-off function in prior hydraulic dampers, or spacers attached to thedamping piston assembly and/or piston rod for limiting maximum extensionor speed of extension of the damper on the rebound stroke, may provideinefficient and undesirable performance in dampers using MR fluids orother fluids.

Providing a hydraulic rebound cut-off feature with a shock absorber formof damper is known. Such a device is disclosed in U.S. Pat. No.2,379,750. That hydraulic rebound cut-off feature uses a rod guidehaving a collar forming an anchorage for an upper end of a coil springwhose lower end is secured to a ring valve. When the piston approachesfull extension, the ring valve is contacted, which closes some fluidpassages completely and others partially to reduce their fluid flowcapacity, increasing damping force and slowing extension directedtravel. This prior art device undesirably restricts fluid flow betweenthe valve and the piston.

Yet another type of known hydraulic rebound cut-off feature utilizes arebound cut-off piston in conjunction with a damping piston. Such adevice is disclosed in U.S. Pat. No. 4,342,447. According to this priorart design, a fixed/clamped disk or disk stack on a secondary or reboundcut-off piston co-acts with the damping piston to effect a substantialentrapment of fluid in the extension chamber of the shock absorber asthe damping piston approaches full rebound. However, in this device, anindentation in the wall acts as a piston stop and not as a support forthe rebound cut-off piston.

Still another type of known hydraulic rebound cut-off feature utilizes arebound cut-off device in conjunction with a damping piston. Such adevice is disclosed in U.S. Pat. No. 5,277,284. According to this priorart design, a spring is held on the damping piston by a retaining ringon the piston rod. However, in this device, the retaining ring does notact as a support for the rebound cut-off device.

In other dampers, the rebound cut-off disk is retained by a snap ring inthe wall of the tube, or is attached to the rod guide by means of aspring. However, the snap ring reduces available stroke, while theattachment is awkward and prone to failure. Still other dampers includea rebound cut-off disk that is heavier than fluid in the tube, and theyare supported by a plastic float. The float requires a certain volumeand, therefore, takes up valuable stroke length. Moreover, in a monotubestrut, the reservoir is upside down, so the float will not stay near therod guide.

Particularly with monotube design dampers, maximizing active length iscritical. This is because a typical monotube damper carries a gas cupthat separates out a gas chamber within the single tube of the device.The gas chamber is expansible and contractible to account for thechanging volume of space occupied by the piston rod entering and exitingthe tube. Presence of the gas chamber minimizes the amount of activelength that can be utilized by other features such as the reboundcut-off device. Therefore, there is a need in the art for a reboundcut-off feature for a monotube damper with minimal impact on damper deadlength.

SUMMARY OF THE INVENTION

According to the present invention, an improved hydraulic reboundcut-off feature of a suspension damper is provided. It can be added to adamper with minimal impact on damper dead length, has fewer parts, islow cost and light weight.

According to this aspect, a suspension damper includes a fluid-filledtube and a damping piston assembly disposed within the tube and slidablymounted therein for reciprocal movement in the tube. A piston rodextends through the tube and connects to the damping piston assembly,and a rod guide assembly closes a bottom end of the tube. The suspensiondamper also includes a rebound cut-off disk suspended in the tubebetween the rod guide assembly and the damping piston assembly. Thesuspension damper further includes a spring suspended in the tubebetween the rod guide assembly and the rebound cut-off disk. The springand the disk have specific gravities greater than the specific gravityof the fluid in the tube such that they sink to the bottom of the tubeadjacent the rod guide assembly. Together, the spring and the reboundcut-off disk cooperate with the damping piston assembly to provide arebound cut-off effect between the rebound cut-off disk and the dampingpiston assembly.

The invention may also take the form of a method for limiting the extentand/or speed of extension of the damper on the rebound stroke by havinga spring and a disk with specific gravities less than the specificgravity of the liquid in the tube to provide a rebound cut-off effectbetween the rebound cut-off disk and the damping piston assembly.

A suspension damper of the present invention incorporates a hydraulicrebound cut-off feature. As a result, a suspension damper of the presentinvention is light-weight, has fewer parts and is relatively low cost.The rebound cut-off effect results in the full damper area having a veryhigh force, yet the compressing loads are essentially zero.

Other features and advantages of the present invention will be readilyappreciated as the same becomes better understood after reading thesubsequent description, when considered in connection with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described, by way of example, withreference to the accompanying drawings, in which:

FIG. 1 is a schematic cross-section of a monotube strut of the presentinvention.

FIG. 2 is a schematic cross-section of the suspension damper housed inthe monotube strut of FIG. 1.

DETAILED DESCRIPTION

FIG. 1 illustrates an exemplary embodiment of a suspension damper 10 ofthe present invention, housed in a monotube strut. It may be understoodthat the suspension damper 10 of the present invention may be housed inany variation of a monotube strut known in the industry.

FIG. 2 depicts an exemplary embodiment of a suspension damper 10,according to the present invention, having a cylinder tube 12 defining aworking chamber 14 for containing a fluid therein, and defining an axis16. A rod guide assembly 18 closes a bottom end of the cylinder tube 12,and is adapted for receiving and guiding a piston rod 20, and forproviding a sliding fluid seal between the piston rod 20 and thecylinder tube 12.

A damping piston assembly 22 is slidably disposed in the working chamber14 for reciprocating motion along the axis 16. The piston rod 20 has afirst end 24 and a second end 26. The first end 24 of the piston rod 20is connected to the damping piston assembly 22, for linear movement ofthe piston rod 20 and damping piston assembly 22 within the workingchamber 14, along the axis 16. The second end 26 of the piston rod 20extends along the axis 16, through the rod guide assembly 18, and out ofthe working chamber 14.

Those having skill in the art will recognize that the componentsdescribed thus far, in relation to FIG. 2, would further include anumber of features known in the art, such as fluid passages and valvecomponents in the damping piston assembly, for example, that have beenomitted from this explanation for clarity. We contemplate, however, thatour invention may be practiced in forms incorporating such featuresknown to those having skill in the art.

The damper 10 furtber includes a rebound cut-off feature, in the form ofa disk 28 having a central bore 30 therein, whereby the disk 28 isdisposed about the piston rod 20 between the damping piston assembly 22and the rod guide 18 for sliding motion of the disk 28 within thecylinder tube 12 and along the piston rod 20. The cylinder tube 12defines an inner surface 13 thereof, and the di5k 28 defines an outersurface 29 thereof, adjacent to the inner surface 13 of the cylindertube 12. The outer surface 29 is spaced from the inner surface 13 of thecylinder tube 12 to form a small gap 31 between the cylinder tube 12 andthe disk 28, thereby defining a generally annular-shaped restriction inthe fluid passage 33, in conjunction with the inner wall 13 of thecylindrical tube 12. The axial length and outer periphery of the reboundout-off disk 28 can be closely controlled and matched to the inner wall13 of the cylinder tube 12 to accurately and conveniently control theeffective flow characteristics of the fluid passage 33, and facilitatetuning of the damper 10 to meet desired performance parameters forrebound cut-off operation.

The working chamber 14 includes a volume of fluid 32 having a specificgravity. The disk 28 is configured in such a manner that the specificgravity of the disk 28 is greater than the specific gravity of thefluid, so that the disk 28 sinks in the fluid 32 toward the bottom endof the working chamber 14, adjacent the rod guide assembly 18, duringnormal operation of the damper 10.

In the first exemplary embodiment, the fluid 32 is a Magneto-Rheological(MR) fluid, but other fluids such as oil are also contemplated. The disk28 is fabricated as a solid circular disk, having a specific gravitygreater than that of the fluid. In an exemplary embodiment, the disk ismade of metal, and is over-molded with a plastic material or elastomericmaterial, such as nylon, such that the specific gravity of the resultingcomponent is more than that of the fluid 32. The elastomer serves toreduce the noise when it impacts the piston. Such materials are robustenough to absorb the physical loads imposed on the disk 28 in arrestingthe motion of the damping piston assembly 22, and heavy enough to sinkin the fluid 32. It may be understood, however, that the materials forthe disk 28 may be varied depending on the type of fluid 32 used in theworking chamber 14, whether an MR fluid or other type of fluid.

Because the disk 28, in a damper 10 according to the present invention,sinks in the fluid 32 to the bottom of the working chamber 14, away fromthe damping piston assembly 22, rather than having a spacer attached tothe damping piston assembly or piston rod as in prior dampers, the fluid32 does not need to flow around the disk 28 during normal, i.e.,non-maximum, extension of the damper 10. This allows the gap 31 betweenthe disk 28 and the cylinder tube 12 to be smaller than would be thecase in prior dampers, even when using highly viscous MR fluids.

Having a small gap 31 between the cylinder tube 12 and the disk 28results in a tighter fit between the cylinder tube 12 and the disk 28that is conducive to reducing operational noise of the damper 10, suchas rattling of the disk 28 in the cylinder tube 12. Having the disk 28float in the working chamber 14 away from the damping piston 22 alsocontributes to reducing operational noise.

The damper 10 of FIG. 2 further includes a conical compression spring36. The working chamber 14 includes a volume of fluid 32 having aspecific gravity. The spring 36 is configured in such a manner that thespecific gravity of the spring 36 is greater than the specific gravityof the fluid, so that the spring 36 sinks in the fluid 32 to the bottomend of the working chamber 14 during normal operation of the damper 10.The spring 36 is disposed between the rod guide assembly 18 and the disk28 for holding the disk 28 away from the rod guide assembly 18, andforming a small reservoir 38 of fluid 32 between the rod guide assembly18 and disk 28. The spring 36 can be fabricated from any material suchthat the specific gravity of the spring 36 is greater than that of thefluid 32.

As discussed above, the gap 31 between the cylinder tube 12 and the disk28 defines a passage 33, which permits fluid flow between the disk 28and the inner surface 13 of the cylinder tube 12 for filling andemptying the small reservoir 38 created by the spring 36 between thedisk 28 and the rod guide assembly 18.

The material and configuration of the disk 28, spring 36, and fluid 32are selected to have respective specific gravities that result in thedisk 28 and spring 36 being sinkable in the fluid 32 at the bottom endof the working chamber 14.

In another embodiment of the present invention, the disk 28 has aspecific gravity sufficiently greater than the specific gravity of thefluid 32 such that the disk 28 and the spring 36 will sink in the fluid32. Thus, in this embodiment, the specific gravity of the spring 36 doesnot necessarily have to be greater than that of the fluid 32, and may infact, be equal to or greater than the specific gravity of the fluid.

During normal, i.e., non-rebound-cut-off, operation of the damper 10,the damping piston assembly 22 is free to move independently from thedisk 28 and its related components, as the piston rod 20 moves in andout of the cylinder tube 12 in response to the motion of the vehiclesuspension. As the damper 10 approaches a maximum extended position onthe rebound stroke of the damper 10, the damping piston assembly 22 isbrought to bear against the disk 28.

Further motion of the damping piston assembly 22 on the rebound stroke,after the damping piston assembly 22 contacts the disk 28, forces thedisk 28 to move downward, compressing the spring 36 such that the coilsof the spring nest inside each other, due to the conical configuration,thereby having a minimum collapsed length of about one wire diameter.The compression of the spring 36 forces fluid trapped below the disk 28in the small reservoir 38 to flow through the passage 33 past therebound cut-off disk 28 in a controlled manner that generates additionaldamping which resists and slows the damping piston assembly 22 tothereby provide a hydraulic rebound cut-off function that reduces impactof the damping piston 22 against the rod guide 18. Once the spring's 36compression is maximized, rebound cut-off operation ceases and the disk28 functions as a spacer, preventing further extension of the damper 10during the rebound stroke.

Following rebound cut-off operation, the spring 36 forces the disk 28away from the rod guide 18, as the damping piston assembly 22 movesupward along the axis 16 on the compression stroke of the damper 10. Asthe disk 28 moves away from the rod guide 18, fluid 32 in the workingchamber 14 flows through the passage 33 and past the disk 28 to refillthe small reservoir 38.

While the present invention has been illustrated by the description ofone or more embodiments thereof, and while the embodiments have beendescribed in considerable detail, they are not intended to restrict orin any way limit the scope of the appended claims to such detail.Additional advantages and modifications will readily appear to thoseskilled in the art. The invention in its broader aspects is thereforenot limited to the specific details, representative apparatus and methodand illustrative examples shown and described. Accordingly, departuresmay be made from such details without departing from the scope or spiritof Applicant's general inventive concept.

1. A strut assembly having a suspension damper comprising: asubstantially vertically-oriented tube having a top end and a bottomend; a rod guide assembly closing the bottom end of the tube; a dampingpiston assembly disposed within the tube and slidably mounted thereinfor reciprocal movement in the tube, wherein the tube is substantiallyfilled with a fluid having a specific gravity that damps thereciprocating movement of the damping piston assembly within the tube; apiston rod connected to the damping piston assembly and extendingthrough the tube and the rod guide assembly; a rebound eat-off disksuspended in the tube between the rod guide assembly and the dampingpiston assembly and cooperating with the damping piston assembly toprovide a rebound cut-off effect between the rebound cut-off disk anddamping piston assembly; and a spring disposed between the reboundcut-off disk and the rod guide assembly, wherein the disk has a specificgravity that is greater than the specific gravity of the fluid, wherebythe disk moves in the fluid toward the bottom end in response to its ownweight.
 2. The strut assembly of claim 1 wherein the spring has aspecific gravity that is greater than the specific gravity of the fluid,whereby the spring sinks in the fluid.
 3. The strut assembly of claim 1wherein the fluid is a magneto-rheological (MR) fluid.
 4. The strutassembly of claim 1 wherein the fluid is oil.
 5. The strut assembly ofclaim 1 wherein the disk comprises metal overmolded with a plasticmaterial.
 6. The strut assembly of claim 1 wherein the disk comprisesmetal overmolded with an elastomeric material.
 7. The strut assembly ofclaim 1 wherein the spring is conical in shape and the coils of thespring nest within each other, thereby having a minimum collapsed lengthof about one wire diameter.
 8. The strut assembly of claim 1 wherein thedisk is circular in shape and has an outer periphery defining an annulargap in conjunction with an inner wall of the tube for fluid flowtherebetween.
 9. The strut assembly of claim 1 wherein the spring has aspecific gravity equal to or less than the specific gravity of the fluidand the disk has a specific gravity that is sufficiently greater thanthe specific gravity of the fluid such that the spring and the disk movein the fluid toward the bottom end and remain near the rod guideassembly in response to the weight of the disk.
 10. A strut assemblyhaving a suspension damper comprising: a tube; a rod guide assemblyclosing a bottom end of the tube; a damping piston assembly disposedwithin the tube and slidably mounted therein for reciprocal movement inthe tube, wherein the tube is substantially filled with a fluid having aspecific gravity that damps the reciprocating movement of the dampingpiston assembly within the tube; a piston rod connected to the dampingpiston assembly and extending through the tube and the rod guideassembly; a rebound out-off disk suspended in the tube between the rodguide assembly and the damping piston assembly and cooperating with thedamping piston assembly to provide a rebound cut-off effect between therebound cut-off disk and damping piston assembly; and a spring disposedbetween the rebound cut-off disk and the rod guide assembly, wherein thedisk has a specific gravity that is greater than the specific gravity ofthe fluid, whereby the disk sinks in the fluid, wherein the diskcomprises metal overmolded with a plastic material or an clastomericmaterial, wherein the disk is circular in shape and has an outerperiphery defining an annular gap in conjunction with an inner wall ofthe tube for fluid flow therebetween, wherein the spring has a specificgravity that is greater than the specific gravity of the fluid, wherebythe spring sinks in the fluid, and wherein the spring is conical inshape and the coils of the spring nest within each other, thereby havinga minimum collapsed length of about one wire diameter.
 11. The strutassembly of claim 10 wherein the fluid is a magneto-rheological (MR)fluid.
 12. The strut assembly of claim 10 wherein the fluid is oil.